A conventional toner for the development of an electrostatic image which has previously been widely used in electrophotography has been prepared by a process which comprises melt-kneading a mixture of a binder resin such as of a syrene-acrylate copolymer, or polyester, a coloring agent such as carbon black and a pigment, a charge controller and/or a magnetic material through an extruder, grinding the material obtained, and then classifying the resulting powder. However, the conventional toner obtained by such a melt-kneading/grinding process has the disadvantage that the controllability of the particle diameter of the toner is limited, making it difficult to prepare a toner substantially having an average particle diameter of not more than 10 μm, particularly not more than 8 μm in a good yield. Thus, the conventional toner cannot be considered good enough to provide the high resolution that will be required in the future electrophotography.
In order to achieve oil-less low temperature fixability, an approach involving the blend of a low softening wax in a toner during kneading has been proposed. In the kneading/crushing process, however, the amount of such a wax to be blended is limited to about 5 w/w %. Thus, toners having a sufficient low temperature fixability cannot be obtained.
In an attempt to overcome difficulty in controlling the particle diameter and hence realize high resolution, JP-A-63-186253 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a process for the preparation of a toner involving emulsion polymerization/agglomeration process. However, this process is limited in the amount of wax that can be effectively introduced into the agglomeration step. Thus, this process leaves something to be desired in the improvement in oil-less low temperature fixability.
Specifically, the present inventors have conducted investigations by increasing the addition amount of wax on the basis of teach of the above-described patent. As the result, it was found that with increasing the addition amount of the wax, the resulting toner undesirably had two peaks in a particle diameter distribution or finely divided powder having a particle diameter of 1 μm or less remained, which required a classification step after the agglomeration step.
In U.S. Pat. No. 5,849,546 and JP-A-10-301322, a so-called capsulated toner is disclosed that is produced by agglomerating primary polymer particles obtained by emulsion polymerization, then fixing a particulate resin on the surface of the resulting agglomerated particles. In this publication, low temperature fixability and high resolution are attempted to be achieved. However, neither primary polymer particles comprising wax encapsulated therein nor particulate resin comprising wax encapsulated therein are used and, therefore, sufficient performance is difficult to achieve.
In U.S. Pat. No. 5,965,316 a toner is disclosed wherein a particulate resin obtained by emulsion polymerization using wax as seed is coated over an agglomerate of particles. In this toner, however, the wax is present in a large amount in the outermost layer thereof and, therefore, the wax leaks out prior to fixing. This can pollute the apparatus with wax residue and is very detrimental to the process.